A population of microorganisms called “activated sludge” are biological solids that play an important role in the biological sewage/waste water treatment process for removing organic matters, nitrogen, phosphorus, etc. from the sewage/wastewater. Efficiency of operation in the sewage water treatment process (i.e., activated sludge process) using an activated sludge is greatly affected by the level of concentration and the amount of the pollutants (including organic matters) in the influent sewage water, the concentration and condition of activated sludge in the reactor, hydraulic retention time, solid retention time, etc. Especially, the concentration of activated sludge is recognized to be a very important factor in a typical activated sludge process as well as an advanced sewage water treatment process including the removal of nitrogen and phosphorus. However, the level of concentration of an activated sludge could not be properly controlled or maintained at a high level by the settling process of the prior art, which is most commonly used among the existing solid-liquid separation processes. Especially, the settling process has several problems involving intermittently occurring sludge bulking, re-floatation of activated sludge due to nitrogen gas generated from the settling tank through the de-nitrification procedure, resulting in a reduction of efficiency of the solid-liquid separation and a loss of the excess activated sludge.
Heretofore, to operate a high biomass concentration of activated sludge process, the followings have been developed: a membrane bioreactor (MBR) process using submerged separation membrane, a pure oxygen aeration process, a media-chargeable biofilter process, which is the representative attached organisms' treatment process, and etc. In activated sludge processes, the maintenance of high concentrations of microorganisms will make an improvement in the processing capacity for organic matters in the aeration tank and, thus, an increase in the loading rate of organic matters (per unit volume), resulting in a possibility of reduction in volume of the aeration tank of up to 50˜75% and of the significant reduction in the required area accordingly. Also, in these cases, the efficiency to remove ammonia nitrogen may be maximized since the nitrification rate will be enhanced by virtue of the increase in the concentration of nitrification microorganisms. Moreover, it is expected that the amount of excess produced sludge is to decrease, since the F/M (food to microorganisms) ratio will be lower and thus self-oxidation will vigorously progress. However, for the processes such as MBR process and biofilter process, the initial cost will be high due to the purchase of membranes or media as well as, in longer operations, the clogging of membranes by an activated sludge or a secretion thereof occurs and thus a loss of pressure increases. Under this circumstance, the membrane should be washed or back-washed periodically. For the aeration process using pure oxygen, it is known that it is impossible to stably operate the aeration process and also it is difficult to maintain a concentration of the microorganisms in a aeration tank of more than 8,000 mg/L since a very good precipitation of activated sludge should be ensured in order to maintain the concentration of activated sludge at a high level. For the aeration process, additional costs are also required to supply pure oxygen.